A variety of exhaust manifolds have been proposed which are reduced in weight and facilitate early activation of a catalyst by controlling temperature decrease in exhaust air. For example, Unexamined Japanese Patent Publication No. 10-89064 discloses an exhaust manifold composed of three sheet metal members, that is, a front half body, a partition body and a back half body, superposed on each other. In the exhaust manifold, a second exhaust pipe and a third exhaust pipe communicated with a second exhaust port and a third exhaust port are formed between the front half body and the partition body. A first exhaust pipe and a fourth exhaust pipe communicated with a first exhaust port and a fourth exhaust port are formed between the partition body and the back half body.
Unexamined Japanese Patent Publication No. 2000-248930 discloses an exhaust manifold for use in a four cylinder internal combustion engine. The internal combustion engine has first to fourth exhaust ports. The order of exhaustion from the first to the fourth exhaust port is the first to the third to the fourth to the second. The exhaust manifold is provided with an outer case including a first branch pipe part, a second branch pipe part, a third branch pipe part, and a collecting pipe part. The collecting pipe part is formed by merging the first to third branch pipe parts. The first branch pipe part is connected to the first exhaust port, the second branch pipe part is connected to the second and third exhaust ports, and the third branch pipe part is connected to the fourth exhaust port, of the internal combustion engine. A partition pipe communicated with the second and third exhaust ports is also provided to extend from the inside of the second branch pipe part to the inside of the collecting pipe part. The partition pipe is opened inside the collecting pipe part.